Teaching Case

A 51-year-old female presented with left knee pain since several months and was posted for left total knee arthroplasty. MRI done a month back showed bone marrow oedema in the lateral tibial condyle. Total leucocyte count (TLC) was 4420 mm³ and C-reactive protein (CRP) was 20.9 mg/L.

A plain radiograph showed a well-defined subarticular lytic lesion in the lateral tibial condyle (Figure 1).

A repeat MRI (magnetic resonance imaging) was done, which showed a well-defined oval lesion in the lateral proximal tibial metaepiphysis. The centre of the lesion was hypointense on the T1WI (T1-weighted image (Figure 2) and hyperintense on T2WI (Figure 3). The penumbra sign was seen with an inner wall, hyperintense on T1WI. The outer wall was hypointense on both T1WI and T2WI. Osteoarthritic (OA) changes were seen with thinning of the articular cartilage and subchondral cystic changes.

There was surrounding perilesional bone marrow oedema, hyperintense on proton density fat-saturated (PDFS) images (Figure 4). Mild synovial effusion was seen (Figure 5).

CT (computed tomography) showed a well-defined lucent lesion with surrounding sclerosis and a breach of the articular surface (Figures 6a and 6b). Post-operative X-rays showed antibiotic beads filling the cavity (Figures 7a and 7b).

Background

Tuberculous osteomyelitis and arthritis generally arise from the reactivation of bacilli lodged in the body during primary infection through a haematogenous route. The infection involves the metaphysis of long bones and vertebral end plates due to their rich vascular supply.

Clinical Perspective

Making a timely diagnosis is difficult as patients present with vague complaints of pain and may not show any signs of systemic illness, unlike pyogenic osteomyelitis. Tuberculous osteomyelitis occurs in the elderly in developed countries but can occur in children and young adults in endemic regions. Pyogenic osteomyelitis and Brodie’s abscess are common in children and young adults.

Imaging Perspective

MRI is the imaging modality of choice for the investigation of osteomyelitis. Bone marrow oedema is the earliest feature of acute osteomyelitis and can be detected as early as 1 to 2 days after the onset of the infection [1].

Tuberculous osteomyelitis can present as a lytic lesion in the metaphysis of long bones with surrounding sclerosis and can mimic Brodie’s abscess. An intraosseous abscess demonstrates the classic penumbra sign on MRI formed by pus in the centre, which is hyperintense on T2, surrounded by granulation tissue, which is hyperintense on T1 and enhances intensely following contrast administration. This sign has been reported to be highly specific for osteomyelitis [2].

Periosteal reaction is usually not a typical feature of adult tuberculous infections [3]. Transphyseal spread of infection into the joint is common in tuberculosis, unlike pyogenic infections. In pyogenic arthritis, joint space narrowing is early and significant, while in tuberculosis, it is late and mild [4].

Outcome

The management of intraosseous abscess consists of curettage, cancellous bone grafting or filling the abscess cavity with antibiotic-impregnated polymethylmethacrylate (PMMA) beads [5]. This is followed by a prolonged course of parenteral or oral antibiotics, and in the case of tuberculosis, administration of antitubercular drugs for 6–9 months or an extended period of 18 months [6].

In our patient, CT-guided aspiration from the tibial lesion was done, which revealed pus. Synovial fluid revealed a total leucocyte count of 95,000 mm³. The culture was negative. Post-curettage bone and synovial tissue histopathology revealed necrosis and multiple confluent and discrete granulomas s/o tuberculous inflammation (Figure 8).

Take Home Message

Not every cause of knee pain in the elderly can be attributed to degenerative changes; imaging helps in making an accurate diagnosis of osteomyelitis or bone abscess.

All patient data have been completely anonymised throughout the entire manuscript and related files.